来源：ACS Publications
Chemically flexible A2B2O7 oxides are attractive candidates for designing structure-specific functional materials by judicious substitution. However, modulating functionality by substitution is governed by the solubility limits of the substituent. Present work reports an a typical 10 mol % solubility of Y3+ in La2Ti2O7(LTO:10Y), substantially lower than 60 mol %, anticipated by cationic radius ratio guidelines (rA/rB) for A2B2O7. Systematic experimental and theoretical investigations have been performed to justify this limited solubility. The solubility of Y3+ in La2Ti2O7 was verified by XRD at the bulk scale and via an Eu probe at the polyhedral level. Whereas XRD shows phase segregation beyond 10 mol % Y3+, the 0.5 mol % Eu3+-codoped LTO:10Y sample showed typical Eu3+ emissions, advocating structural homogeneity. However, the corresponding excitation spectrum revealed that insertion of Y3+ in LTO enhanced the energy transfer by the titanium–oxygen polyhedral network, [TiO6]8– → Eu3+. These observations were rationalized through density functional theory (DFT) calculations, which unveiled yttrium’s preference for a low-coordination La site that substantially modifies adjacent TiO6 polyhedra and influences the energy transfer. Combined experimental and theoretical studies propose that yttrium’s preference for a lower coordination number and a shorter/stronger metal–oxygen bond renders site selectivity and simultaneous regulation of the TiO6 network, limiting its solubility in La2Ti2O7. Thus, it provides supporting evidence for bonding-controlled substitution of yttrium in La2Ti2O7, rendering solubility lesser than that endorsed by radius ratio guidelines.